Magnetic Resonance Imaging (MRI) provides medical diagnostic information that can be difficult or impossible to obtain otherwise. This information often requires that the electrocardiogram (ECG) of the patient be monitored or measured during imaging. One reason for monitoring ECG signals is to synchronize the acquisition of images with the heart beat to, for example, reduce motion artifacts associated with the heart beat in the acquired images and to obtain dynamic images of the heart. Or, the patient may be so ill that his or her vital signs need to be monitored during the imaging procedure. Further, the effect of pharmacological stressors can be monitored via ECG signals. And, as medical science improves, other uses will be developed as well.
The ECG itself is a small electrical signal, on the order of millivolts, created by the nerves and muscles in the heart. The ECG signal is commonly sensed or detected by placing electrodes on the patient's chest. Wires from the electrodes carry the small electrical signals to an amplifier and other circuitry for ultimate display for health care professionals and for use by the MRI imaging system, for example, in synchronizing image acquisition in conjunction with the patient's heart beat.
However, the MRI equipment itself provides a very difficult environment for the measurement of these small signals. The wires are subject to changing magnetic fields, which can induce potentials in the wires that are orders of magnitude larger than the ECG. This noise or interference makes it difficult for the health care professional or the MRI imaging system to readily observe or monitor the patient's ECG.
The MRI equipment generates high power radio frequency (RF) magnetic fields, the exact frequency depending upon the steady state magnet strength (B0) and the atom being imaged. Generally the RF signals are in the megahertz (MHz) range. MRI scanners also include gradient coils that create magnetic field gradients across the volume of interest in the patient being imaged. Gradient fields are essential to the creation of the image. They can be switched on the order of a thousandth of a second, creating signals in the kilohertz (kHz) range. In addition, as stated above, the MRI scanner generates a very large steady state magnetic field (B0). As the wires are slowly moved through this field due to patient breathing or other motion, the change in flux can induce signals in the wires. While these signals are not steady voltages, the change can be so slow that the frequency is in the single digit Hertz (Hz) range.
The noise generated by and induced on the ECG signal by the RF magnetic field can be handled or removed from the ECG signal. High resistance leads, capacitors, and shunting diodes are commonly applied to the ECG acquisition circuitry to handle the RF magnetic field noise.
Several approaches are described in previous patents for addressing the gradient field noise issue, which is the most problematic in revealing the underlying ECG signals. Moore in U.S. Pat. No. 4,991,580 uses a slew rate limiting (SLR) circuit to reduce the gradient pulse amplitude while not reducing the ECG signal amplitude. Tsitlik et al in U.S. Pat. No. 5,217,010 utilize a secondary low pass filter or a band reject filter to similarly reduce the amplitude of the gradient noise pulse. Gober in U.S. Pat. No. 5,052,398 uses two fourth-order Butterworth filters. One is a low pass filter with a cutoff of 20 HZ. The second is a high pass filter with a cutoff of 10 Hz. An absolute value function filter then follows this Butterworth filter arrangement. Kreger et all in U.S. Pat. No. 5,436,564 use three (3) adaptive filters that receive an input from the MRI system's 3 gradient coil drivers. Blakeley et al. in U.S. Pat. No. 4,991,587 also utilize adaptive filtering. In a second patent, U.S. Pat. No. 5,038,785, Blakeley et al. utilize a sample and hold technique to remove the gradient pulses. The sample and hold technique works by recognizing that a gradient noise pulse is starting to occur and then holding the current ECG value until the gradient noise pulse has passed.
While each of these approaches may have been sufficient at the time they were invented, MRI systems continue to advance and improve. In the early 1990s, a temporal period on the order of seconds was typically required to acquire one MRI data set, and the gradients might only be switched a few times in that interval. Driven by the desire to have faster scans and acquire different types of information, there are significant trends toward faster gradient coil switching speeds. This increases the amplitude and frequency of the induced noise or interference. The other trend is to have the gradient coils activated or changed more frequently. Two examples of current sequences are FIESTA by General Electric Co. and FISP by Siemens, Inc. FIESTA has pulses with components in the kHz range with very fast rise time. In addition, perfusion imaging utilizes streams or bursts of gradient pulses.
These more “aggressive” protocols mean that solutions such as a sample and hold will begin to fail because there is less time to take a sample and more time required in the hold mode. The performance of the various strategies will also degrade as the number and amplitude of the gradient pulses increases. Specifically, the circuit of Moore in U.S. Pat. No. 4,991,580, the contents of which are hereby incorporated by reference, has a frequency response that has a higher gain for signals in the kilohertz range than at DC. This can cause it to function sub-optimally as gradient pulses become faster and more frequent.